EUROPEANSOUTHERNOBSERVATORY

Organisation Europ´eennepour des Recherches Astronomiques dans l’H´emisph`ereAustral Europ¨aische Organisation f¨urastronomische Forschung in der s¨udlichen Hemisph¨are

OBSERVING PROGRAMMES OFFICE • Karl-Schwarzschild-Straße 2 • D-85748 Garching bei M¨unchen • e-mail: [email protected] • Tel. : +49-89-32 00 64 73 APPLICATION FOR OBSERVING TIME PERIOD: 87A

Important Notice: By submitting this proposal, the PI takes full responsibility for the content of the proposal, in particular with regard to the names of CoIs and the agreement to act according to the ESO policy and regulations, should observing time be granted

1. Title Category: A–6 Spectra of Gravitationally Lensed by Abell 2744, the Largest Separation “”

2. Abstract / Total Time Requested Total Amount of Time: 0 nights VM, 9 hours SM Violent cluster mergers, when observed in the plane of our sky, place constraints on the physical nature of particles. These events are rare, and Abell 2744 is one of the best examples. This cluster merger is similar to the “Bullet Cluster” but more massive and with an even larger (the largest observed) separation between dark matter and the stripped, shocked cluster gas. Chandra X-ray observations of A2744 reveal extremely high temperature (> 15 keV) shock regions. We have obtained new deep multiband HST/ACS images of A2744’s core to derive detailed dark matter maps. Our analyses reveal 30+ arcs produced by strong gravitational lensing. Spectroscopic of these arcs are essential to determine the masses of the merging clumps and constrain DM collsionality. None have been published to date. Therefore we are requesting 9 hours on VIMOS, primarily to obtain spectroscopic redshifts of strongly lensed arcs produced by this cluster. 3. Run Period Instrument Time Month Moon Seeing Sky Mode Type A 87 VIMOS 9h sep n 1.0 CLR s

4. Number of nights/hours Telescope(s) Amount of time a) already awarded to this project: b) still required to complete this project:

5. Special remarks:

6. Principal Investigator: Dan Coe, [email protected], US, Jet Propulsion Laboratory

6a. Co-investigators: B. Frye Other,US F. Braglia University of British Columbia,Department of Physics and Astronomy,CA R. Dupke Observatorio Nacional, MCT, CNPq,BR R. Massey Institute for Astronomy, The University of Edinburgh, Royal Observatory, Edinburgh,UK Following CoIs moved to the end of the document ... 7. Is this proposal linked to a PhD thesis preparation? State role of PhD student in this project

- 1 - 8. Description of the proposed programme

A – Scientific Rationale: The “Bullet Cluster” (1E0657-56) provides “direct empirical evidence for the existence of dark matter” (Markevitch04, Clowe04, Clowe06) and upper limits on the DM particle’s self- 2 collisional cross section: σ/m ∼< 1cm /g (Randall08). This cluster merger appears to have followed this “simple” scenario: dark matter and galaxies passed directly through the collision while the hot cluster gas was shocked, stripped back, and left behind. This clearly reveals the different self-collisionalities of the various components, invalidating the simplest MOND/MOG hypotheses that DM is nothing but “missing mass” or “stronger gravity”. As strong as this evidence is, the Bullet Cluster is but a single system. Additional cluster mergers must be studied, but these events are rare, and only a handful have been studied to date. The “Baby Bullet” (MACS J0025.4-1222; Bradac08) does appears to follow the Bullet Cluster scenario. However, analyses of two z ∼ 0.8 clusters (CL0152-14 and MS1054-03) seem to reveal galaxies slightly trailing the dark matter (with gas behind further still; Jee05a,b). And the initial analysis of Abell 520 (the “Cosmic Train Wreck”; Mahdavi07) suggested an opposite scenario, in which dark matter is stripped but gas is not! More recently, Abell 2146 was shown to have an X-ray structure very similar to the Bullet Cluster including a prominent bow shock, yet strangely the BCG trails the X-ray gas plume (Russel10). It seems that cluster mergers do not all follow a simple scenario, and additional analyses are warranted. The Abell 2744 cluster merger (z = 0.308) exhibits the largest observed separation between dark matter and X-ray gas (5400 ∼ 240 kpc, Shan10). Gas does appear to be stripped and left behind, though it has a complex temperature distribution along with a secondary gas clump residing to the NW. This NW gas clump may be unrelated, though recent work (Boehringer06, Braglia07,09) strongly suggests it is trailing a still infalling NW subcluster which is about to join the merger of two other clumps already in progress. Meanwhile, weak lensing analysis of VLT/FORS1 images (updated from the analyses presented in Cypriano04) show that the SE galaxies are slightly trailing the SE DM mass clump (Fig. 1). Again we find deviations from the simple scenario of gas being stripped from DM and galaxies. To map the DM structure of A2744 in more detail, we have obtained deep multiband Hubble Space Telescope imaging. Strong lensing analysis of these images reveals 30+ multiple images of 10+ background galaxies (Fig. 2). Similar to the Bullet Cluster analyses (Bradac06), we will obtain a robust mass model through simultaneous modeling of the strong + weak lensing observed in the HST and wider fields (here, VLT/FORS1). Our SL+WL analysis method (SaWLens; Merten09) has been demonstrated to yield reliable reconstructions of complicated mass distributions including cluster mergers similar to A2744. This method was applied to VLT/FORS and VLT/ISAAC data of MS2137-24. Spectroscopic redshifts for one or more the lensed arcs are essential to normalize our mass model, specifically, the mass of each merging subcluster. Arc uncertainties can translate into uncertainties of a factor of ∼ 2 in the mass of each subclump (Bradac06). Currently no spectroscopic redshifts have been published for arcs lensed by A2744 (although see §10a and Fig. 4 right). Our ACS+VLT strong+weak lensing analyses including spectroscopic redshifts will provide robust and detailed maps of the DM in A2744’s core. By comparing the positions of the DM clumps, galaxies, and X-ray gas clumps, we will help trace the merger history of this cluster. Furthermore, A2744 has a higher mass than the Bullet Cluster, offering the potential for tighter constraints on the dark matter self-interaction cross section. Ancillary science (spectra to be obtained in parallel): We will increase the completeness of spectral coverage in the central regions of the cluster (Fig. 4 left), including fainter cluster members (I ∼> 22). This will provide us with more precise estimates of: 1) the dynamical mass (Biviano06; to be compared with the lensing and X-ray derived masses and used in joint analyses); 2) kinematics in the high line-of-sight velocity core, revealing how the velocity distribution is affected by the ongoing merger; 3) blue fraction as a function of magnitude / stellar mass (Couch98). We will also target some especially blue cluster members to investigate possible star-formation activity enhance- ments in cluster galaxies due to harassment (e.g., the “flaming giants” detected by Braglia et al. 2007) and/or ram pressure stripping. One especially enigmatic galaxy worthy of further study is highlighted in Fig. 3.

B – Immediate Objective: We will obtain multislit spectroscopy at R = 600 to measure redshifts for a sufficiently bright subset of our ∼ 30 images of ∼ 10 galaxies strongly lensed by Abell 2744. We will also study the spectral features of the IGM along the lines of sight to the arcs (Frye07,08). In parallel, we will obtain spectra of cluster galaxies as described above. We will include blue cluster galaxies, planning to measure the star formation rates, ISM gas turbulence and physical conditions based on the line fluxes, line widths, and line profiles of the available emission lines. Regarding our Fig. 3 galaxy, NTT/EMMI data reveal [OIII]λ5007 and Hβ (Boschin06: ID90). We will also search for additional Balmer series lines to deduce the reddening as well as [OII], [OIII], [SiII] and [NII] to better understand the physical and evolutionary state of this dynamically-disturbed .

- 2 - 8. Description of the proposed programme and attachments

Fig. 1: Abell 2744 exhibits the largest observed separation Fig. 2: Based on strong lens modeling of (~54′′ ~240 kpc; Shan10) between dark matter (shown here in our HST/ACS images, we have identified blue) and hot cluster gas (red, from Chandra) overlaid on our 30+ multiple images of 10+ background newly-obtained HST/ACS images (BVi color image, 3.5′ × 2.8′ objects. Spectroscopic redshifts of these section). The dark matter map is an updated version of that arcs are essential to determine mass of this obtained via weak lensing analysis in Cypriano04. The (SW) clump as well as normalize the full galaxies at left appear to be slightly trailing the dark matter. mass model. The other clumps exhibit This would be interesting, as hints of this have been seen fewer arcs, but their spec-z will be equally elsewhere (Jee05a,b). We are improving this DM map important. Spec-z have yet to be obtained significantly by including strong and weak lensing constraints for any A2744 arcs (although see Fig. 4). which we have obtained from HST/ACS data (Fig. 2). The HST/ACS BVi image stamp: 100′′×100′′ bright X-ray source is featured in Fig. 3. (1′′ ~ 4.3 kpc).

Fig. 3: This galaxy appears to be undergoing prominent ram pressure stripping and star formation as it plows through Fig. 4: Left – Many A2744 cluster galaxies remain without confident the cluster gas. It is also a very spectroscopic redshifts. Overlaid on the VLT/FORS1 VRI and HST/ACS bright X-ray source, likely BVi color images are objects with published redshifts in the central ~7′×7′ indicating a central AGN (or from Braglia09 VLT/VIMOS (spec-z, squares) and Boschin06 perhaps intense star formation). NTT/EMMI (mostly SED fitting, smaller circles): green = cluster Analysis of our deep, high member; cyan = foreground; red = background. resolution (R = 600) spectra will Right – A 2002 FORS2 campaign (69.A-0407(A)) obtained spectra for reveal whether the gas was these objects near the SE BCG, including perhaps 2 or 3 of our arcs. shocked into forming stars and Analysis of these data has yet to be published by the observers, but we then stripped, or vice versa. have begun to analyze them ourselves.

Braglia09 A&A 500,947 • Shan10 MNRAS 406,1134 • Bradac06 ApJ 652,937 • Randall08 ApJ 679,1173 • Bradac08 ApJ 687,959 • Jee05a ApJ 618,46 • Jee05b ApJ 634,813 • Mahdavi07 ApJ 668,806 • Russel10 1004.1559 • Boehringer06 Msngr 123,49 • Cypriano04 ApJ 613,95 • Merten09 A&A 500,681 • Couch98 ApJ 497,188 • Frye07 ApJ 665,921 • Frye08 ApJL 685,5 • Boschin06 A&A 449,461

- 3 - 9. Justification of requested observing time and observing conditions

Lunar Phase Justification: We require new moon as this is a redshift survey with important emission features that can appear anywhere in the bandpass. Time Justification: (including seeing overhead) We estimated the exposure times using the latest ESO Spectroscopy E.T.C. version 2.14, with the following parameters:

• blackbody spectrum • no AO

• seeing=0.80 arcsecs • airmass=1.50 • 200 milliarcsec resolution scale • DIT=900 sec.

We require a signal-to-noise >5, leading to exposure times on targets of 2.25hrs per mask for a total of three multislit masks. Including 20 min per hour of observation to account for overheads, we request a total observing time of 9hrs. Whenever a bright Natural Guide Star is available nearby, we will used AO with NGS. Details per targets are provided in box 12. We therefore request a total of 9 hours including overheads to study ∼ 20 strongly-lensed arcs.

9a. Telescope Justification: VIMOS is uniquely suited for this project as this source is not accessible from northern sites (Mauna Kea or La Palma). The unique combination of multiplexing power and large area offered by VIMOS, combined with the light-collecting power of the VLT, assure that we will be able to maximize the number of targets with each exposure, while minimizing number of exposures and overheads in between.

9b. Observing Mode Justification (visitor or service): This program is well-suited for SM given the single target.

9c. Calibration Request: Standard Calibration 10. Report on the use of ESO facilities during the last 2 years 081.A-0387/SINFONI (PI: B. Frye): Data are fully reduced, analysis is in progress. One paper is in prepa- ration.

10a. ESO Archive - Are the data requested by this proposal in the ESO Archive (http://archive.eso.org)? If so, explain the need for new data. A2744 was targeted as part of a 2003 FORS2 campaign (69.A-0407(A)) to obtain spectra of cluster lensed arcs. Analysis of these spectra has yet to be published by the observers, but we have begun to examine the data ourselves as they appear to include two or three of our arcs (Fig. 4 right). Even if these spectra yield reliable spec-z, our observations should yield improved spectra for these objects as well as for many of our other 20+ arcs revealed only recently by our HST images and lens modeling. VLT/VIMOS (Braglia07,09), NTT/EMMI (Boschin06), and FLAMES-GIRAFFE spectra have been obtained for bright cluster members. As part of our ancillary science, we will target fainter cluster members at higher resolution (R = 600 compared to R = 200 for the previous VIMOS observations).

10b. GTO/Public Survey Duplications:

11. Applicant’s publications related to the subject of this application during the last 2 years Braglia et al. 2009, A&A 500, 947: Multi-wavelength study of X-ray luminous clusters at z ∼ 0.3 Frye, B. et al., 2008, ApJL 685, L5: Observations of the Gas Reservoir Around a Star Forming Galaxy In The Early Universe Massey, R. et al., 2010, astro-ph/1007.1924, Cluster Bulleticity Coe, D. et al., 2010, ApJ, in press, A High Resolution Mass Map of Substructure: LensPerfect Analysis of Abell 1689 Merten J. et al., 2009, A&A, 500, 681: Combining weak and strong cluster lensing: applications to simulations and MS 2137 Meneghetti M. et al., 2010, A&A, 514, 93: Weighing simulated galaxy clusters using lensing and X-ray Zitrin, A., Broadhurst, T. et al., 2010, astro-ph/1002.0521, Strong-Lensing Analysis of a Complete Sample of 12 MACS Clusters at z > 0.5: Mass Models and Einstein Radii

- 5 - 12. List of targets proposed in this programme Run Target/Field α(J2000) δ(J2000) ToT Mag. Diam. Additional Reference star info

A A2744 00:14:16.1 -30:22:58.8 9 25 200 z=0.308

- 6 - 13. Scheduling requirements

- 7 - 14. Instrument configuration Period Instrument Run ID Parameter Value or list

87 VIMOS A MOS-slits-targets 0.600 < slit width < 1.400, tar- gets:extended

- 8 - 6b. Co-investigators: ...continued from box 6a. J. Krick Spitzer Science Center,California Institute of Technology,US E. Cypriano Universidade de Sao Paolo,Instituto de Astronomia, Geofisica e Ciencias At- mosfericas,Department of Astronomy,BR L. Sodr´eJr. Universidade de Sao Paolo,Instituto de Astronomia, Geofisica e Ciencias At- mosfericas,Department of Astronomy,BR A. Zitrin Tel Aviv University,Department of Astronomy & Astrophysics,IL J. Merten University of Heidelberg,Department of Physics and Astronomy,D N. Ben´ıtez Instituto de Astrofisica de Andalucia (IAA),E T. Broadhurst Other,E M. Meneghetti INAF - Osservatorio Astronomico di Bologna,I Y. Jim´enez-Teja Instituto de Astrofisica de Andalucia (IAA),E J. Bregman University of Michigan,Astronomy Department,US

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